The present invention pertains to the field of reactors and methods for electrochemically treating integrated circuit substrates, and in particular, to the shaping of electric fields to control electric current density on substrates during electrochemical treatment.
Statement of the Problem
A crucial component of integrated circuits is the wiring or metalization layer that interconnects the individual circuits. Wiring layers have traditionally been made of aluminum and a plurality of other metal layers that are compatible with the aluminum. In 1997, IBM introduced technology that facilitated a transition from aluminum to copper wiring layers. The transition from aluminum to copper required a change in process architecture (to damascene and dual-damascene), as well as a whole new set of process technologies. Copper damascene circuits are produced by initially forming trenches and other embedded features in a wafer, as needed for circuit architecture. These trenches and embedded features are formed by conventional photolithographic processes. Usually, a barrier layer, e.g., of tantalum or tantalum nitride, is formed on silicon oxide in the embedded features. Then, an initial xe2x80x9cseedxe2x80x9d, or xe2x80x9cstrikexe2x80x9d, layer of copper about 1250 xc3x85 thick is deposited by a conventional vapor deposition technique. The seed layer should have good overall wafer uniformity, good step coverage (in particular, a continuous layer of metal deposited onto and conforming to the side-walls of an embedded structure), and minimal closure or xe2x80x9cneckingxe2x80x9d of the top of the embedded feature. See, for example, xe2x80x9cFactors Influencing Damascene Feature Fill Using Copper PVD and Electroplatingxe2x80x9d, Reid, J. et al., Solid State Technology, July 2000, p. 86.
The seed layer is used as a base layer to conduct current for electroplating thicker films. In plating operations, the seed layer functions initially as the cathode of the electroplating cell to carry the electrical plating current from the edge zone of the wafer, where electrical contact is made, to the center of the wafer, including through embedded structures, trenches and vias. The final thicker film electrodeposited on the seed layer should completely fill the embedded structures, and it should have a uniform thickness across the surface of the wafer. Generally, in electroplating processes, the thickness profile of the deposited metal is controlled to be as uniform is possible. This uniform profile is advantageous in subsequent etch-back or polish removal steps.
Any change in conditions that increases the seed layer""s resistivity or the seed layer""s electrical path will exacerbate the difficulty of achieving a uniform current distribution, which is necessary for effective global electrofilling and uniformity. A number of industry trends, however, tend to increase the seed layer resistivity. These include 1) thinner seed layers, 2) larger diameter wafers, 3) increased pattern density and 4) increased feature aspect ratio (xe2x80x9cARxe2x80x9d). Unfortunately, these trends produce challenging conditions for electrofilling, and are not generally amenable to maintaining uniform current density across a wafer. For example, for a given PVD seed deposition condition, smaller features are substantially more xe2x80x9cneckedxe2x80x9d as compared to larger features. As the feature size shrinks, the fixed necking amount becomes relatively more restrictive of the etched feature opening. This effect causes the effective aspect ratio (that is, the AR of the feature into which the plating process must begin plating) of the smaller width features to be substantially higher than that of the original, unseeded etched feature. In order to minimize the necking effect, a thinner seed layer with more conformal side wall coverage is desirable. However, a thinner seed layer causes the initial current distribution across the wafer to become more non-uniform, which (if left uncompensated) leads to poor electrofilling uniformity across the wafer. The seed layer initially causes significant resistance radially from the edge to the center of the wafer because the seed layer is thin. This resistance causes a corresponding potential drop from the edge where electrical contact is made to the center of the wafer. Thus, the seed layer has a nonuniform initial potential that is more negative at the edge of the wafer. The associated deposition rate tends to be greater at the wafer edge relative to the interior of the wafer. This effect is known as the xe2x80x9cterminal effectxe2x80x9d.
Thus, industry trends create a need for increasingly thinner seed layers having uniform thickness. It is anticipated that in the near future, seed-layer thickness will decrease to below 500 xc3x85, and may eventually decrease to as little as 100 xc3x85. Decreased seed layer thicknesses, combined with increased wafer diameters, however, require improvements in hardware and methods to maintain uniform electroplating.
Various studies have shown the importance of thin seed-layer properties, feature aspect ratio, and feature density on initial plating uniformity. U.S. Pat. No. 6,027,631, issued Feb. 22, 2000, to Broadbent et al., which is hereby incorporated by reference, teaches using asymmetrical shields to influence plating current.
U.S. Pat. No. 6,132,587, issued Oct. 17, 2000, to Jorne et al., teach various methods of mitigating the terminal effect and improving the uniformity of metal electroplating over the entire wafer, including increasing the resistance of the electrolyte, increasing the distance between the wafer and the anode, increasing the thickness of the seed layer, increasing the ionic resistance of a porous separator placed between the wafer and the anode, placement of a rotating distributor in front of the wafer, and establishing contacts at the center of the wafer. Jorne et al. disclose a xe2x80x9crotating distributor jetxe2x80x9d that directs different amounts of flow to different radii of a wafer. Creating a spatially varying flowrate at the wafer to influence the global current distribution is practically difficult because the conditions of plating locally vary (flowrate, replenishment of additives, etc.) and, therefore, create a difficult-to-separate convolution between electrofilling and uniformity. Futhermore, no practical means of controlling plating conditions with respect to process time and film thickness was disclosed.
A general approach has been discussed of using a highly electrically resistive membrane placed in close proximity to the wafer so as to establish a xe2x80x9cthin resistive platingxe2x80x9d region where the potential drop across the wafer will be always smaller than the system potential drop. While this approach might work theoretically, in practice there are a number of problems. Firstly, placing the membrane close to the wafer is difficult (distance between membrane and wafer is typically about 1 cm or less for a typical copper acid plating bath having a conductivity of about 500 ohmxe2x88x921 cmxe2x88x921). Secondly, the potential drop and, therefore, the required power increase greatly. Also, establishing uniform flow to the wafer is difficult with a highly restrictive membrane so close to the wafer. That is, it is hard to decouple the fluid flow and the electric field problems because the membrane does not only resist current flow, but also resists fluid flow that needs to be directed at the wafer to replenish consumed reactants.
The ability to successfully electrofill (i.e. the ability to electroplate very small, high AR features without voids or seams) is dependent on a number of parameters. Among these are the 1) plating chemistry, 2) feature shape, width, depth, and density, 3) local seed layer thickness, 4) local seed layer coverage, and 5) local plating current. Items 3-5 are interrelated. As an example of this convolution, a decrease in seed-layer thickness can lead to greater potential differences between the center and edge of a wafer, and hence larger variations in current density during plating. Additionally, it is known that poor seed layer side-wall coverage leads to higher average resistivities for current traveling normal to the feature direction (for example, in trenches), also leading to large current density differences between the center and edge of a wafer. It has generally been observed (independent of plating chemistry) that effective electrofilling occurs only over a finite range of current densities. And while the appropriate electrofilling current density can depend on such things as feature shape, feature width or plating chemistry, for any given set of these parameters, there is typically a finite range of localized current density in which electrofilling can be successfully performed. Therefore, an apparatus and a method for plating at a uniform current density over a whole wafer are needed.
Another problem is the difficulty of achieving globally uniform electrodeposition and electrofilling in large diameter wafers. The industry has recently made a transition from 200 mm wafers to 300 mm wafers. Electrofilling generally requires that the current density increase proportionately with the wafer diameter. Thus, a 300 mm wafer requires 2xc2xc times more current than a 200 mm wafer. It has been shown that the resistance from the edge to the center of the wafer is independent of radius. See, Broadbent, E. K. et al., xe2x80x9cExperimental and Analytical Study of Seed Layer Resistance for Copper Damascene Electroplatingxe2x80x9d, J. Vac. Sci. and Technol. B17, 2584 (November/December 1999). With greater applied current at the edge (to maintain the same current density), the potential drop from the edge to the center of the wafer is correspondingly greater in a 300 mm wafer than in a 200 mm wafer. Therefore, there is a need for an apparatus and a method that compensate for the potential drop across the wafer, which changes during electroplating.
Defects at the very edge of electroplated wafers are common. Air bubbles, and to a much smaller extent particulates, often become trapped on the wafer surface, during the immersion of the face-down wafer. The defect-causing bubbles and other agents tend to form or accumulate at the edge of the wafer. Also, plating solution can become trapped in the region of the contacts seal. This can result in corrosion of the seed layer at the outer periphery of the wafer.
Therefore, it would be useful to have available an apparatus and method for electroplating a uniform, relatively thin layer of metal (for example, less than 7000 xc3x85) on an integrated circuit wafer having a thin seed layer (for example, less than 500 xc3x85) with no defects out to the periphery of the wafer (for example, within 2.5 mm of the wafer edge).
The invention helps to solve some of the problems mentioned above by providing systems and methods to achieve superior uniformity control and improved electrofilling of wafers having 1) thinner seed layers, 2) larger diameter (e.g. 300 mm instead of 200 mm), 3) higher feature densities, and 4) smaller feature sizes.
In one aspect of the invention, an apparatus for electrochemically treating the surface of a substrate comprises a plurality of dynamically operable concentric anodes opposite a substrate holder. In another aspect, a diffuser shield is located between the substrate holder and the concentric anodes. In another aspect, an insert shield is located between the diffuser shield and the substrate holder.
In aspect of the invention, an apparatus for electrochemically treating a surface of a substrate comprises a first bath container operably configured to retain an electrochemical bath at a bath height. In another aspect, a plurality of separately operable concentric anodes is disposed in the first bath container. In another aspect, a substrate holder is disposed in the first bath container opposite the concentric anodes at a substrate height. In still another aspect, a shield is disposed in the first bath container between the concentric anodes and the substrate holder, the shield operably configured for shielding a surface area of a substrate when a substrate is held in the substrate holder during electrochemical treatment operations. In another aspect, an embodiment in accordance with the invention includes a means, operable during electrochemical treatment operations, for dynamically varying a parameter selected from the group consisting of: a quantity of shielded surface area of a substrate, a distance separating the shield from the substrate holder, a distance separating the substrate holder from the concentric anodes, and combinations thereof. Another aspect is a variable weir assembly for dynamically varying the bath height and an actuator for dynamically moving the substrate holder, to vary dynamically the substrate height. In still another aspect, the first bath container has a first overflow height, and a second bath container surrounds the first bath container and has a second overflow height higher than the first overflow height, and a third, overflow container surrounds the second bath container. Another aspect of the invention is a first valve for maintaining an electrochemical bath at the first overflow height, and a second valve for maintaining an electrochemical bath at the second overflow height. In another aspect, an apparatus includes a movable sluice gate in the bath container wall for controlling the bath height. In still another aspect, the shield is a diffuser shield located between the concentric anodes and the substrate holder. In another aspect, the diffuser shield comprises a plurality of rings rotatable about a common axis, each of the rings configured to have an open area and a closed area. In another aspect, an embodiment in accordance with the invention includes an actuator for dynamically rotating one of the rings to vary the open and closed areas and, thereby, a quantity of shielded surface area of a substrate. In another aspect, the shield is an insert shield located between the anode and the substrate holder. In another aspect, the insert shield is separated from the substrate holder by a flow gap. Another aspect is a movable spacer for attaching the insert shield to the substrate holder and an actuator for moving the spacer to vary dynamically the flow gap. In another aspect, an apparatus further includes means for rotating the substrate holder.
In another aspect, a diffuser shield has an inside lip diameter in a range of about from 8 inches to 12 inches. In still another aspect, the diffuser shield is a beta-type diffuser shield having wedge-shaped open areas in an annular lip. In another aspect, an insert shield has an inside diameter in a range of about from 10.5 to 12 inches. In another aspect, the insert shield and the substrate holder form a flow gap having a width in a range of about from 0.075 to 0.3 inches. In another aspect, the insert shield has a streamline-type rim portion. In still another aspect, the insert shield has a modified streamline-type rim portion having a radius of curvature in a range of about from {fraction (1/16)} to one-half inch.
In one aspect of the invention, a method for electrochemically treating the surface of a substrate comprises steps of providing an electrochemical bath with an anode located at the bottom of the electrochemical bath, placing a wafer substrate in the substrate holder, and then immersing the wafer substrate held in the substrate holder into the electrochemical bath opposite the anode. In another aspect, a method includes a further step, prior to the step of immersing, selected from the group consisting of: pre-washing an electrical contact in the substrate holder, and pre-wetting the wafer substrate. A further aspect is a step of rotating the wafer substrate.
In another aspect, a method for electrochemically treating the surface of a substrate comprises steps of immersing the wafer substrate into the electrochemical bath at a substrate height and opposite the concentric anodes. Another aspect is a step of providing a diffuser shield located between the wafer substrate and the concentric anodes. Another aspect is a step of providing an insert shield located between the diffuser shield and the wafer substrate. Another aspect of the invention is dynamically varying the power delivered to the concentric anodes. Another aspect is a step of dynamically varying the flow gap between the insert shield and the substrate holder. In another aspect, an embodiment in accordance with the invention comprises a step of dynamically varying a closed area of the diffuser shield. In still another aspect, an embodiment comprises steps of dynamically varying the bath height, and dynamically varying the substrate height.
In one aspect, a method for electrochemically treating a surface of a substrate comprises steps of dynamically varying a parameter selected from the group consisting of a quantity of shielded surface area of the substrate, a distance separating the shield from the substrate, a distance separating the substrate from the concentric anodes, and combinations thereof. In a further aspect, embodiment comprises steps of dynamically varying the bath height in the first bath container, and dynamically moving the substrate holder, to vary dynamically the substrate height. In another aspect, a method comprises steps of substantially closing a first outlet valve so that electrochemical fluid substantially fills a second bath container, thereby generating a second bath height, and controlling a second valve in a third container to maintain the second bath height. In another aspect, an embodiment comprises steps of dynamically moving the substrate holder to vary the substrate height, thereby actuating a movable sluice gate in a bath container wall for controlling the bath height. In another aspect, the shield is a diffuser shield comprising a plurality of rings rotatable about a common axis, each of the rings configured to have an open area and a closed area, and the diffuser shield is located between the concentric anodes and the substrate holder, and a method further comprises dynamically rotating one of the rings to vary a quantity of shielded surface area of a substrate. In another aspect, the shield is an insert shield attached to the substrate holder by a movable spacer and located between the anode and the substrate holder, and a method further comprises steps of actuating the movable spacer to vary dynamically a flow gap between the insert shield and the substrate holder.
In addition to being useful in a wide variety of electroplating operations, embodiments in accordance with the invention are generally useful in numerous types of electrochemical operations, especially during manufacture of integrated circuits. For example, embodiments are useful in various electrochemical removal processes, such as electro-etching, electropolishing, and mixed electroless/electroremoval processing.
Embodiments in accordance with the invention are described below mainly with reference to apparati and methods for electroplating substrate wafers. Nevertheless, the terms xe2x80x9celectrochemical treatmentxe2x80x9d, xe2x80x9celectrochemically treatingxe2x80x9d and related terms as used herein refer generally to various techniques, including electroplating operations, of treating the surface of a substrate in which the substrate or a thin film of conductive material on the substrate functions as an electrode.
The terms xe2x80x9cdynamicxe2x80x9d, xe2x80x9cdynamically variedxe2x80x9d and similar terms herein mean that a variable or parameter of an apparatus or method is selectively changed during the treatment of a wafer. In particular, a variable or parameter is dynamically varied to accommodate the changing electrical properties of a deposited metal layer as layer thickness increases (or decreases in layer removal treatments) during electrochemical treatment operations. The term xe2x80x9ctime-variablexe2x80x9d and similar terms are used more or less synonymously with terms such as xe2x80x9cdynamicxe2x80x9d.
The term xe2x80x9cdynamically operablexe2x80x9d used with reference to a device generally means that the function or operations of the device can be selectively changed during electrochemical treatment of a particular substrate. The terms xe2x80x9cdynamically operablexe2x80x9d, xe2x80x9cseparately operablexe2x80x9d and similar terms used with specific reference to concentric anodes are used in two senses. In one general sense, the terms mean that one or more concentric anodes of a plurality of concentric anodes in a given electrochemical treatment apparatus can be controlled in a circuit including a power supply and a cathodic wafer substrate separately and independently from other concentric anodes. In a second general sense, the terms mean that two or more concentric anodes of a plurality of concentric anodes are connected in parallel to a power supply, and the total power delivered by the power supply can be selectively distributed between the connected concentric anodes.